There are known highly unsaturated fatty acids or the derivatives thereof such as the esters thereof which originate from fish oils such as oils of sardines, mackerels, tunas or the like. Especially, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and the esters thereof have been used in the field of health foods and drugs because of their pharmacological effects.
The known processes for the purification of these unsaturated fatty acids or the derivatives thereof are a urea-addition process, a precision distillation process, a chromatographic process, a supercritical fluid extraction process and the like. However, these prior processes are individually difficult to purify a mixture comprising highly unsaturated fatty acids or their derivatives originating from fish oils, in high purity, without modification, in large quantity and at low cost. In the urea-addition process, the resultant mixture comprising highly unsaturated fatty acids or their esters is low in a degree of purity. In the precision distillation process, polymerization and isomerization occur with easy modification of the product and it is difficult to remove eicosatetraenoic acid (ETA) or its ester which is not desirable when used as drugs and foods. Further, the chromatographic and supercritical fluid extraction processes are not suitable for the separation and purification on an industrial scale.
Japanese Patent Kokai 9-151390 discloses a process for the purification of a highly unsaturated fatty acid and the derivative thereof, which comprises contacting a mixture of highly unsaturated fatty acids or their derivatives with a carrier carrying a silver salt, and subsequently separating and extracting by contacting the carrier with a solvent. It is described therein that silica gel, zeolite, kaolin, active china clay, pearlite and the combination thereof are used as a carrier. However, this publication gives no reference to the use of diatomaceous earth and the effect achieved thereby. Further, this prior process gives the limitations that the amount of the carrier for the silver salt should be 19 parts by weight in terms of silver relative to 45-55 parts by weight of the carrier, and further, that the amount of the mixture of highly unsaturated fatty acids or their derivatives to be contacted with the silver salt-carried carrier should be 15 parts by weight. The departure from such limitations results in lowering a degree of purification. Although the prior process can separate and purify a highly unsaturated fatty acid component in high purity, there are several problems that a process for preparing the silver-carried carrier is complicated, and that an extraction operation including repeated stirring and filtering is troublesome.
There has been a demand for a practical process of selectively separating and purifying desired EPA and DHA from fish oils containing various highly unsaturated fatty acids and their derivatives.